I think the answer is two-fold. First of all, I think she was displaying a typical emotional response: denial. Sexual abuse victims often have emotional reactions that don't seem to the rest of us to make sense or be the most logical reactions. Especially when the aggressor is someone that they have previously trusted, victims often tend to deny that anything happened or that that person is a threat. They also usually feel emotions like shame, humiliation, and guilt. It is common for sexual abuse victims to try to rationalize what has happened or to try to appear to others as if everything is normal. To dis-invite Spike would have required an explanation, one that Buffy probably did not feel ready to provide. I don't think she wanted to admit -- to herself or to anyone, but especially not to Dawn -- what had happened.
Second, in Buffy's case, I think these typical feelings of guilt may have been exaggerated because she had some very legitimate reasons to feel guilty and to blame herself. She had abused Spike physically and verbally for the duration of their relationship. Violence was an integral part of sex for them, most of which was initiated by Buffy. I think part of her saw Spike's reaction as a reflection of her own darker nature. A lot of their season 6 relationship was about Buffy learning to come to terms with the dark side -- the demonic, monstrous, not all hearts and butterfly hair clips side -- of herself. Because she could not accept that side of her own nature, she could not accept it in Spike, and so she punished him physically and verbally as a means of punishing and devaluing herself. Unfortunately, the natural response of a creature who has been beaten and abused and rewarded for doing only one thing (that thing being sex) is to do that one thing. To expect him to do otherwise would be to admit that he is more than just a thing, something that Buffy was never willing to do.
In this scenario, Spike is being viewed by Buffy as sort of like Pavlov's dog; his negative reaction was, to her, her fault because she used the wrong conditioning stimuli. I think part of Buffy's continued trust in Spike after the incident in the bathroom is that she viewed that incident as avoidable in the future if only she changed the conditioning. Basically, he was to her a thing, no more human than the Buffybot, but programmable in a different way. He was a creature or a thing who had, in her estimation, only behaved as she had trained it to. If you starve a dog and train it to attack on command in return for food, you can't blame the hungry dog when it attacks in an attempt to get food. If you program the Buffybot to attack all demons on sight, you can't blame it when it attacks poor Clem on his way home from the grocery store. In much the same way, I don't think Buffy blamed Spike right away because I don't think she thought he could be held accountable as a human. Instead, she was the human in the situation, and so she felt that she was the one to be blamed.
